main(List<String> args) {
  // 循环语句
  // for for-in
  List list = [1, 2, 3, 4, 5];
  for (var i = 0; i < list.length; i++) {
    print(list[i]);
  }

  print('--------------------');

  for (var item in list) {
    print(item);
  }

  print('--------------------');

  Map map = {'first': 1, 'second': true};
  // 错误 为空
  for (var i = 0; i < map.length; i++) {
    print(map[i]);
  }

  print('--------------------');

  // 不能直接for-in循环map，使用map.keys map.values
  for (var item in map.keys) {
    print('$item: ${map[item]}');
  }
}
